Variable resistor



March 29, 1932. N. c. SCHELLENGER VARIABLE RES I STOR Filed March 5, 1931 3 Sheets-Sheet JWe/zfw" C/ZeZZQ/ye/ March 29, 1932. c SCHELLENGER 1,851,933

VARIABLE RESISTOR Filed March 5, 1951., F s Sheets-$11961. 2

Patented Mar. 29, 1932 2 UNITED STATES PATENT OFFICE I NEWTON C. SCHELLEN GER, OF ELKHART, INDIANA, ASSIGN OR TO CHICAGO TELEPHONE SUPPLY COMPANY, OF ELKHART, INDIANA, A CORPORATION OF INDIANA VARIABLE RESISTOR Application filed March 5, 1931. Serial No. 520,287.

My invention relates to rheostats and the like and is more particularly concerned with a method of and means for making contact between two electrical conductors. The

: novel principle herein first disclosed may be applied in a wide variety of situations and mechanisms, as will be apparent from an understanding of the specific device herein disclosed.

Also, specifically, I have herein disclosed a novel form of potentiometer rheostat which, in addition to the broad novelty in method and principle which I shall point out, embodies novel specific constructions constituting detailed improvements.

The particular form of device which I shall discuss and disclose is a potentiometer type of rheostat such as is employed in connection with circuits employing thermionic tubes.

Rheostats employing conducting materials deposited upon a base of paper or cardboard are known. They may be constructed to embody very large values of resistance in small space. It is known to shift a movable contactor along the resistance itself, so that for any change in position of the point of contact a change in the value of the resistance is accomplished.

As a consequence of such large values in small space, the progression of the shiftable contactor along the resistance member must be made as smooth and gradual as possible, otherwise the variation in resistance will not be smooth and the value of resistance will jump from one value to another.

Movement of a wiping contactor similar to those used on wire wound rheostats, directly upon the resistance element has been found to be unnecessarily severe upon the life of the composition resistance element. Such violent movement as is secured by the direct rubbing of the contact arm upon the resistance is, therefore, generally avoided. To overcome the same, various schemes of providing a rolling or rocking contact or engagement by motion normal to the surface, between the shiftable conductor and the resistance, have been evolved.

One such scheme employs a metal roller, 59 rolling on a cylindrical resistance strip which may be disposed on either the outside of a cylinder or upon the inside of the same. Another scheme for avoiding the abrasive effect of the shiftable contactor engaging the surface of the resistor is to lay down and take up a thin strip of metal over the surface of the resistor. Still another scheme employs a metal cone, rocking on a resistance element.

I have studied the action of such forms of rheostats under repeated operation, i. e., under life test, and have observed that long before there is any perceptible wear the rheostat fails to give a satisfactory control of resistance, the changes of resistance being irregular and capricious. The rheostat then causes noises in a controlled radio set, and must be replaced.

I have discovered that this irregularity of action arises from the character of the contact which such rheostat makes, i. e., the rocking or rolling contact. There are several ways in which the rocking or rolling contact upon such a resistance may become defective:

(1) Oxidation of the contacting surface of the rocking or rolling member. This may, to some extent, be avoided by the employment of a non-corrosive-metal for the enga ing surface.

(2) Particles of dust, etc., coming between the rolling member and the resistance element, causing the rolling member to be held away from the resistance element and to make imperfect contact. This can, to some degree, be ameliorated by increasing the pressure of the rocking or rolling contact member upon the surface of the resistance element, but at the expense of other troubles. Also it can, to some degree, be prevented by enclosing the unit.

(3) Due to corrosive effects on the contactors surface that can scarcely be discerned and occasionally a slight imperfection existing on the contacting surface of the contacting member and in still other cases due to a poor type of resistance element, minute particles of the resistance element will adhere to the metallic contacting surface and once started, these places usually grow in area and height rapidly as the unit is operated, this being especially true during humid weather. Shortly after such a condition has first occurred, such irregular control will be had with the rheostat that it must be replaced. As yet no metal suitable for a contactor has been found that will not give trouble from corrosion unless means are provided either to keep the surface polished or to operate the contactor in a vacuum, in an inert gas, or an inert liquid.

I have conceived a novelmethod of and means for avoiding the difiiculties encountered by the above two methods of securing change of resistance through varying the position of a contact, which is different from each of the above. My method, while avoiding the difficulties of each of the above methods, secures benefits over either or both in a marked degree.

In order to acquaint those skilled in the art with my improved method of making contact between two conductors, and a specific means by which the same may be performed, I shall 7 describe the same in connection with the accompanying drawings, in which:

Figure l is a view in perspective of a device illustrative of my method of making shiftable contact between two conductors;

Figure 2 is a diagrammatic representation of the action which takes place between two conductors when my method of making contact is utilized;

Figure 3 is a view in section of a rheostat comprising one embodiment of my invention;

Figure 4 is a view in perspective, in eX- ploded relation, of the parts of the rheostat shown in Figure 3;

Figure 5 is a plan view of the rheostat shown in Figure 3 with the cover removed to show the details of construction of the device;

Figure 6 is a view in section of a double, or tandem type of rheostat embodying my method of making contact;

Figure 7 is a plan view of the tie used for holding the two parts of the rheostat shown in Figure 6 together;

Figure 8 is a plan View during the process of manufacture of a pressure arm suitable for the rheostat shown in Figures 3, 4, 5 and 6;

Figure-9 is a view in section of the contacting portion of the pressure arm shown in Figure 8;

Figure 10 is a view of the contacting portion of the completed pressure arm;

Figure 11 is a view in sect-ion of the pressure arm shown in Figure 5, taken along the line 1111 of Figure 5; and

Figure 12 is a View in section of the rheostat shown in Figure 5 and taken substantially along the line 1212 thereof.

Briefly stated, my novel method consists in engaging the resistance, which may be either a film resistor or a wound resistor, by a contactor, the surface of which, as it is applied to the resistance by pressure normal to the face of the resistance, is at the same time shifted or moved in a direction at right angles to the line of pressure, i. e., substantially in the plane of the surface to be engaged, by a small but definitely controlled amount. In brief, I cause the contactor to wipe along the surface of the resistor at the same time that the point of tangency between contactor and resistor occurs.

Tlfe preferred embodiment of my method of making contact between twoconductors, illustrated in Figure 1, comprises a thin, wafer-like contact member C which is pressed into engagement with and simultaneously shifted or caused to flow along the surface of the resistance member R. The degree of shift or slip of the contact member may be definitely controlled by the proportions, but in any particular embodiment should be great enough to produce a definite troweling,

or smoothing effect upon the surface of the resistance, and a polishing or burnishing effect upon the metal contact member.

In said preferred form the exposed surface of the resistance R with which the contact member engages is disposed in the shape of a truncated cone. Where a film resistance is employed the base strip may thus be shaped as atruncated cone. The contactor C is formed substantially in the shape of a fiat disc, which is flexed into engagement with the conical surface of the resistance element by a swinging arm P. The contactor might be shaped as a cone of less sharpness than the resistance, since all that is needed to secure the desired method of contacting is a sufficient difference in effective circumference of the two parts to cause the definite and controllable slippage between the two parts.

The desired method of operation may be secured by means of a rolling contact memher -which is compelled to move through either greater or less angular motion than would be secured by pure rolling motion between the contactor and the resistance. This may be done, for example, by gearing the two members together upon a different pitch circle than the circle of travel of the surface of the contactor.

There are numerous ways in which this method may be embodied in rheostats. The practical advantage is that a rheostat constructed according to my invention has a useful life greatly in excess of any other form of rheostat now known. The slight shifting effect of the contactor trowels down lightly the surface of the resistance and if any particle of dust is interposed the same is given an opportunity to embed itself in the resistance, under the troweling action of the contactor, without ill effect.

The surface of the metal does not become caked with any part of'the resistor nor does it tend to become oxidized, because the operation of shifting the position of the contactor causes the surface of the resistance to be smoothed down, on the one hand, and the contactor to be polished, or burnished, on the other.

The amount of shift which is required to gain the advantages above recited is small indeed. In the preferred embodiment, wherein the diameter of the rheostat is about 1.75, the contactor shifts along the surface approximately .049. This may, of course, be varied without departing in any way from my invention.

In conjunction with the above I may cause a definite motion of rotation or translation of the contactor to occur at each reversal of motion of the actuating member, to insure a more extended wiping action at any point at which the contactor has been resting. I do this, furthermore, to gain a beneficial action adjacent each terminal of a potentiometer type of rheostat, namely, to secure an especially low cutout resistance at either or both extreme positions of the rheostat. The added wiping or pressing down action on the resistance and the burnishing of the contactor adjacent the end positions is beneficial. It is secured by a very simple expedient, namely, by allowing for a small degree of lost motion of the contactor.

In respect to the specific details of construction, I have provided a novel form of easing which permits of a reduction in the dimensions of the rheostat. The central part of the mounting case is concaved, providing a recess in which a clamping nut may be mounted. This is possible because of the conical shape of the resistor and the novel flexing action of the disc.

The contactor element, although spoken of as a disc, resembles more closely a mutilated wheel. It has a rim portion, a hub portion, and spoke portions, to provide functionally differentiated parts. The rim is wide enough to engage the width of the resistance element by deflecting or bending of the spokes. The hub portions is a suitable means for clamping and mounting the contactor substantially coaxially with the shaft upon which the wiper arm is mounted. The spokes serve the dual purpose of holding the rim away from the resistor and also of resisting the rotary motion which the arm tends to impart to the contactor-that is, the spokes compel the rim portion to pass, or flow backward under the contacting finger to secure the slipping, or wiping effect heretofore noted.

I have endeavored, in Figures 1 and 2, to illustrate diagrammatically the wiping action which occurs between two conductors such as the resistor and contactor of a rheostat, when they are connected according to my method. In practice the resistance element extends out in vertical alignment with the edge of the disc, but in Figure 2 the contact e ement is P will be bent or flexed shown as of no greater radial extent than will be covered by the contactor when it is flexed into engagement. In the upper portion of Figure 2 the contactor element 0 has been shown substantially in the position it occupies with respect to the resistor R when the pressure arm P is pressing against one portion of the contactor. The portion of the contactor directly beneath the pressure arm downwardly against the surface of the resistor. A definite wave of deflection, illustrated to better effect in Figure l, is produced in the contactor.

For the purpose of illustration suppose the arm is being rotated in the direction indicated by the arrow (Figure 2). The circumferential length of the contactor segment is greater than the-circumferential length of the resistor segment with which this contactor would ordinarily make contact, and if no slippage of the contactor on the resistor surface occurred the contactor would occupy successive positions on the resistor, as indicated by the dot and dash lines, that is, the contactor would actually overlap the resistor by a distance indicated by the letter D. But the contacting surfaces of the contactor and resistor are smooth and the arms of the contactor, by reason of their stiffness in the plane of the contactor, and because they are held in place by the hub, which is clamped against angular motion, prevent any rotative movement of the contactor as a whole. Since the contactor cannot rotate it must, and does, slip beneath the pressure arm as it is rotated, thus causing the desired wiping action.

The wave motion of the contactor causes the arms to be moved downward and to be twisted slightly about their longitudinal axis at the same time. This results in movement of any given point G on the contactor in a loop G which may vary in configuration but which is characterized by one feature, Via, a marked and highly useful movement at the surface of the resistor contrary to the direction of movement of the pressure arm as it passes over the point G. It is this movement, or wiping action, which is so beneficial.

The lower portion of Figure 2 is an elevational projection of the upper portion of the figure and is a schematic representation of the action which occurs when the contactor is bent down on the resistor B. This portion of Figure 2 lends itself readily to illustration of a geometrical explanation of how wiping action occurs. The explanation is as follows:

Assume that the line ()C represents the radius of the outer periphery of the disc; the circumference of the circle described by the edge of the disc is equal to 2.7r.O-C. But

in depressing the contactor into contact with the resistor, the edgeof the contactor, whose radius is OC, is caused to occupy successive points in a circle whose radius is OC and whose circumference equal 2m times the radius O'C.

As all points around the circle of the edge of the disc must at some time be in contact with some point on the circle of the edge of the resistor, it is apparent that the slippage in rotating the arm through 360 will just equal the difference in circumference of the two circles, or

The slippage 21rO'--C or The slippage equals 21r(OC minus O' C), or

The slippage equals 21rB-C.

This same condition holds true for all radial contact points beneath the wiper arm as it moves and therefore slippage and the consequent polishing action must occur at all points beneath the wiper arm. In my preferred embodiment the central point does not lie in register with the apex of the cone, and instead of the contactor being gripped at the central point only, it is gripped at the rather extensive hub position. The cutting out of the parts of the disc and leaving only the spokes relieves the necessity of carrying the wave of flexure completely to the center of the contactor, while leaving sufficient resistance to rotary shifting of the rim relative to the hub; also, these spokes are proportioned to give the desired spring lifting or raising effect upon the rim of the contactor.

It is quite apparent, from the foregoing explanation, that a change in the angle BOG between the contactor and the resistor, that is, a change in the slope of the cone, will result in a change in the degree of slippage which occurs, since the effective radius OC of the resistor ischanged, thereby changing the value BC. Also raising the plane of the disc with respect tothe' resistance member as by placing washers under the hub of the disc will increase the amount of slippage of the contacting member relative to the resistance member.

If the angle of the cone is more acute a higher value of slippage occurs; if it is more obtuse a lower value of slippage occurs. I have found that for the preferred form of rheostat herein shown a slope of 1Q A is satisfactory, but it is quite apparent that the slope may vary with different materials and different uses of'my method of making contact. It is apparent also that the disc C may also be a frustum of a cone of less sharpness than the resistor R since it is the difference in the effective radii of the two conductors which produces the slippage. It is quite apparent, therefore, that changes in the dimensions and angles of devices embodying my method of making shifting contact between two conductors are well within the scope of my invention.

Figures 1 and 2 are illustrative of still another feature of the invention. In the deequals 2 nQ-C minus vices of the prior art contact was made either between a fiat surface and a curved surface or between two curved surfaces internally tangent one to the other. In my device a sectional view through the contactor and resistor when they are in contact shows that the contactor presents a convex surface to the resistor because of the shape of the wave of deflection of the contactor as it is bent down under the pressure arm. Since the resistor is a frustum of a cone it also presents a convex surface to the deflected part of the contactor andthe result is that the zone of contact between contactor and resistor is made between two convex surfaces and is narrower and sharper than in devices of the prior art. This permits a more sensitive control and a more effective resistance variation per unit of angular movement of the pressure finger.

In the device illustrated in Figures 3 to 6 inclusive, I provide a cone shaped base 10 of moulded insulating material having a peripheral cylindrical flange 11 constituting a housing or shell 12, in which the working parts of the rheostat may be enclosed as by means of cover 13, which may be secured to the shell in any desired manner. Because the base 10 is cone shaped its outer face may be concaved, as hereinbefore pointed out, in order to provide a recess in which a lock washer 14 and a clamping nut 15 may be mounted. The washer and clamping nut clamp the flanged sleeve or bushing 34 to the housing 12. The threaded sleeve 34 forms a convenient means for mounting the device upon a support or panel as by means of the nut 63 and washer 64. A saving in axial space is thereby effected over the devices of the prior art, particularly devices of the type in which a plate or cone rolls on a flat, circular resistor. In these devices when one side of the contactor is moved down into contact with the resistor the opposite side is tilted upwardly. In my device when movement of a given point on the contactor occurs the opposite side does not move upwardly, consequently only about one half the usual axial space is required for movement of the contactor.

The resistor 21 is preferably in the form of a flexible strip which carries the conduc tive materials. It is disposed upon the truncated cone base and cemented thereto under pressure with a cement being preferably heat resisting. This insures the resistor presenting a smooth even surface of the proper form to the contactor under all normal conditions in which the resistor is intended to operate. The composition and form of the resistor may be varied to meet any particular requirements. Flat seats 22 and 23 which have top surfaces lying in planes tangent to the conical surface of the base are provided on the inner wall of the base. The ends 24: of the resistor strip are provided with openings and the ends of the strip are secured to the inclined faces by means of rectangular plates 25 which are held in place as by rivets 26. These plates 25 are made of springy stock and are preferably slightly concaved toward the ends 24 of the strip 21 to insure positive contact with the resistance element at the edge of the plate in order to provide low minimum resistance. The rivets 26 extend through the base and con stitute means for securing the resistor strip terminals 27 and 28 in recesses in the outer wall of the base.

An annular, axially extending flange 31 is preferably provided-at the central portion of the base in order to afford a centering means for the disc 32 and a washer 33 and to afford an insulation between disc 32 and thimble 34. The base is also provided with a central opening 35, through which the sleeve or bushing 34 projects. A key or projection 36 is preferably provided on the flange 31, adapted to engage suitable notches 37 and 38 in the central openings 39 and 40 of the disc 32 and washer 33, respectively, and a notch 41 in the flange 49 of bushing 34 in order to prevent these parts from turning with relation to the base.

If desired, however, I may make the notch 37 in contactor 32 a trifle wider than the proj ection on base 10. The result is that the contactor 32 is permitted to rotate to a very limited degree and when the direction of movement of the pressure arm is reversed it carries the contactor with it, thus insuring a more extended wiping action at any point at which the contactor has been resting and particularly near the terminal plates 25. Another beneficial action which results is that the cutout resistance is lowered at the extreme positions of the rheostat. Where this lost motion is provided the terminal portion 45 of the contactor is given a suitable flexibility, as by arching the same.

The disc 32 is preferably comprised of a hub portion 42. spokes 43 and a rim 44. The spokes are preferably tapered from the center to the rim and are graduated in width so that deflection of the disc under a given pressure will be substantially the same at any point about the circumference. Since a portion of the wheel is cut away to correspond with the terminal plates-on the resistor strip, and in order to afford a connector tab 45, it is apparent that the greatest stiffness would be required near the unsupported ends of the rim 44 and consequently the greatest width of the'spokes is provided at this point.

A raised portion or block 46 is provided on the inner wall of the base between the projections 22 and 23, to which the tab or terminal 45 may be secured as by means of arivet 47. The same rivet is utilized to fasten a terminal 48 in a depression or recess in the exposed face of the base.

If desired, however, the washer 33 may be made of a conducting material to form an electrical connection between the disc 32 and the thimble 34 and the thimble, instead of the lug 48, used as a terminal for the contactor disc. In a preferred construction the metal. panel 62, such as shown in Figures 3 and 6, is used as a conductor to be attached to the thimble and thus make a connection with the contactor disc.

The washer 33 and disc 32 are secured in place on the base by drawing the flange 49 of the sleeve 34 down against them by means of the nut 15 which is threaded on suitable male threads on the sleeve and which bears against the outer face of the base. The pressure or wiper arm is preferably comprised of a shaft 52, adapted to be ournaled in the bushing 34, having an annular groove 53 therein and anarm 54 secured to the end of the shaft 52 as by heading over the s layed end of the shaft. The arm 54 is pre erably provided with lateral projections 51 adapted to strike the block 46 at the limits of the arm to prevent overtravel.

The arm preferably follows the outline of the inside face of the base, but is offset upwardly at its outer end in order that a wiper pad 55 may be secured to the underside of the arm. This wiper pad may take a variety of forms. In the form shown in Figures 3, 4, 5 and 11, the pad is formed of a rectangular block of material such as oil impregnated leather. A rectangular portion 56 at the end of the arm 54' is punched upwardly to form a rectangular opening 50 in which the leather pad is inserted. Further upward movement of the pad is prevented by the upstruck lug 56 and the pad is held in place by the pressure of the arm against the contactor face.

In the form shown in Figures 8, 9 and 10 the outer end of the arm 54b is enlarged and rectangular openings 57 are provided at the ends of the arm. A strip 55a of leather or the like is inserted in the openings and the sides of the end of the arm bent back upon themselves to fasten the strip securely in .place to constitute a wiper or pressure pad.

The arm 54 is, in each case, made of spring'y material, and it is held in place against the face of the base by means of a washer or snap ring 61 which engages the groove 53 in shaft 52 and bears against the outer end of the bushing 34. A thrust washer 60 may be disposed between the snap ring 61 and the end of the bushing 43 if desired lVhen the cover 13 has been engaged on the base 10 the working parts of the rheostat are fully enclosed and protected against the entrance of particles of dust or the like and against mechanical injury. The rheostat may be fastened to a supporting framework 62 by means of a second nut 63 which is threaded on the sleeve 34 and screwed down until the plate 62 is tightly engaged between the nut 15 and the nut 63. If desired a lock washer 64 may be disposed between the plate 62 and the'nut 63. The shaft 52 is preferably of sufiicient length that it projects through the panel 65 of the radio set or other device with which the rheostat is used. The rheostat may be regulated in any desired manner as by means of the knob 66 which is engaged over the outer end of the shaft 52 and is secured thereto by means of a set screw 67 which engages a suitable flattened portion 68 on the shaft.

My preferred form of rheostat construction lends itself quite readily to the tandem or double type of rheostat construction shown in Figure 6, in which a second or duplicate rheostat structure is placed over the open end of the first rheostat in place of the cover 13.

The structure of the tandem rheostat is substantially the same as the structure of the single rheostat just described, except that a second wiper arm 54a is fastened to the shaft 52 in opposed relation to and overlying the first arm 54 and a cap 10a is mounted on the base 10 in place of the cover 13. The cap consists of a shell member 12a which is of substantially the same construction as the base 10 except that an inside flange 71 is provided at the edge of the shell to coact with an outside flange 72 of the base 10. The cap 10a is prevented from rotating by a keying projection 73 on the base which engages anotch in the flange 71 of the cap.

A resistor strip 21a is located on'the inner face of the cap 12a in the same manner as the resistor 21 is mounted on the inner face of the base 10. A second disc 32a is secured to the central inner face of the cap 12a by means of a bolt 34a which is of the same general form as the sleeve 34 except that it is shorter and is not hollow. The bolt 34a is fastened in place in the cap by means of a nut 15a and lock washer 14a in the same manner as the thimble 34 is secured to the base 10 by means of nut 15.

Any desired means may be utilized in securing the'cap on the base, but I prefer to utilize a strip of fiber 82 (Figure 7) having openings at the center and at each end thereof adapted to encircle the two shells and hold them together. In fastening the cap to the base the strip 82 is wetted and the central opening passed over the thimble 34 to a position against the nut 15. The cap is located in place on the base and the two end openings of the strip are passed over the end of the bolt 34a and a nut 83 and washer 84 are-clamped down upon the ends of the strip. On drying, the strip will contract and hold the cap tightly upon the base. After drying. the strip 82 is lacquered or varnished to exclude moisture which if it entered would loosen theband.

The same saving in space secured in the base is effected in the cap construction because the inner base of the cap is cone-shaped and the outer face may consequently be concaved. Since the wiper arms 54 and 54a move in unison it is apparent that adjustment of the knob 66 will secure adjustment in u11ison of the contactors on the complementary resistor strips.

The operation of the device hereinbefore described is as follows: Assume that the movable arm is at the limit of its throw, with one of the projections 51 engaging the block 46. The end portion of the rim 44, under the wiper pad 55, is pressed into engagement with the resistance 21 immediately adjacent the corresponding terminal plate 25, thereby cutting out substantially all of the resistance as between the corresponding terminal 27 or 28, as the case may be, and terminal 48, which latter is connected to the contactor 32.

Assume now that the notch 37 in the central opening of the hub of the contactor 32 is slightly larger than the key 36, so that a small degree of lost motion is permitted, then the initial motion of the wiper arm causes a slight angular shifting of the contactor 32 upon the resistance member 21. This gives a direct wiping action adjacent the terminal, and away from the same. As soon as the lost motion is taken up, further shifting of the arm and wiper along the rim 44 results in a wave of deflection being carried along said rim against the upward or lifting effect of the spokes 43. Since the contactor 32 cannot move angularly with the wiper, continued movement of the wiper at every phase of its motion results in two actions, namely, a downward pressure of the corresponding part of the rim 44 and a flow or escape of the rim 44 under the wiper in a direction backward with respect to the motion of the wiper.

If at any point in its travel the motion of the wiper is reversed, in the embodiment having the play in the notch 37, the first mos tion in reversal of the wiper will result in I flow or escapement of the contactor under the wiper in the reverse direction continues as above described.

Thus the lost motion connection above described superposes an additional wiping action upon the inherent wiping action which the contactor has by virtue of its inability to move rotatably as it is applied to the smaller conical surface of the resistor.

In the construction where the contactor 32 is rigidly clamped and held against any motion of rotation as a whole, the lost motion above described not being employed, any motion of the wiper results in immediately shifting the point of contact, with the corresponding amount of flow or escapement of the conta'ctor under the wiper. IVhile I have shown a piece of rawhide leather, impregnated with a neutral grease or oil, as being the preferred means for pressing the contactor against the resistance element 21, I might employ a metal to metal engagement along the surface of the rim ll or I might use a small, anti-friction roller, or any other desired way of reducing the resistance to motion while applying the contactor 32 to the resistance and securing the above desirable limited and controlled movement of the surface of the contactor upon the resistance element.

The use of rectangular, resilient pressure pad 56 of oiled leather, applying pressure across substantially the full width of the contacting member 32, has a peculiar utility in the present construction. The resistor 21 is relatively hard and non-compressible for the forces employed therein. The contactor 32 is thin, flexible and resilient. The application of the thin resilient contactor 32 to the relatively rigid resistance element 21 through the resilient leather pressure pad of a substantial area provides an entirely new mode of operation. I am aware that it is old to apply suflicient pressure to a contacting member to compress the resistance member in order to secure contact, but my improvement differs therefrom in that the take-up for variations between the two engaging members occurs within the pad rather than within either of the electrically conducting members. Of course, I do not limit the invention to this particular feature, since other features of the invention herein disclosed are independent of the above.

Referring to the graph shown in the middle of the bottom part of Figure 2, this graph illustrates the motion of a point on the rim of the contactor 32. As the wiper approaches the region of the particular point, the point first moves almost vertically downwardly, as shown by the graph. Then it moves away from the approach of the wiper, as shown by the graph. That is, assume that in the lower part of Figure 2, the wiper is approaching the point from the right, moving towards the left, the particular point G. the motion of which we are studying, first descends substantially vertically, then moves downwardly and towards the left until it is relatively close to the surface of the conical resist-or. Then it reverses its motion, and in the latter part of its approach to the conical surface it travels relatively rapidly in the reverse direction, that is, from left to right. so that when the wiper passes directly over the point it is in engagement with the surface of the resistor and moving relatively rapidly in a direction opposite to the direction of motion of the wiper.' If the arm rests directly upon the point G it of course remains stationarily in contact with the resistor, but as soon as the arm moves further, the point G will be rapidly thrown further to the right to form the loop shown in the graph, and then, as the motion of the wiper further progresses, the point will travel back towards its initial position, the latter part of its motion being substantially vertical. As soon as the wave of deflection has left it the point remains in the horizontal plane of the main body of the contactor 32.

If, while the wiper was resting directly upon the point, the motion of the wiper has been reversed, then the point would trace the first part of the graph on its return to normal position.

The extent or degree of wiping action may be controlled by using wiper pads of different contacting areas. If a pad of larger area is used the degree of wiping action is greater because the point under discussion remains in contact or engagement through a greater number of degrees of movement of the wiper arm, with the result that a relatively greater part of the motion occurs when the two parts are in engagement. If the pad is of smaller area conversely a wiping action of lesser degree occurs.

I am aware that it is old to rub two surfaces together-in order to secure a good electrical contact between them but I believe it is broadly new with my present invention to secure a continuous controlled wiping motion between a contact member which has no motion of translation or rotation as a whole, and a cooperating surface which is stationary. I believe it is also broadly new in rheostats of the present type, employing fragile or film resistance, to secure the aforesaid troweling action by lost motion, and it is broadly new, so far as I am aware, to secure the troweling action by the escapement of the contactor member under the wiper which applies it to the film surface.

While I have described the invention in connection with a potentiometer form of rhcost-atit is not to be limited to that embodiment. Also, while I have described the invention in connection with an amorphous resistance body or film resistance, the same advantage of a controlled wiping action can be secured by the use of my invention even though the contactor is to be used in connection with a wound metallic resistance.

I do not intend to be limited to the details shown or described. as my invention extends beyond the same, although there is considerable novelty in the details themselves and the specific embodiment shown.

I claim:

1. In a rheostat. a resistor formed as the frustum of a cone. a flat contacting member normally located in a plane above the resistor and held against rotation and means for progressively bending a portion of the contacting member into contact with the resistor.

2. A rheostat comprising a resistor having one face lying in a conoidal surface, a flat eontacting'member secured against rotation and located in a plane above the resistor and a wiper'for progressively bending down portions of the contacting member into contact with the resistor.

3. A device of the character described including an electrical conducting element of relatively high resistance lying in the surface of a frustum of a cone, a second normally flat electrical conducting element of relatively low resistance disposed above the frustum and secured against rotation, and means for bending down progressive points of said second element into contact with the first element.

4. A rheostat comprising a housing having an axially recessed concave base, the inner surface of said base forming a convex support for a resistance element, a mounting panel for said rheostat, a bush ing extending recessed outer face, a contact member located the rethrough,

substantially at the apex of the cone'and holding means for holding the contact member in place extending from the contact member through the base and located in the recess of the outer face.

6. In a device of the class described, a cone-shaped base having a central opening a non-rotatable contacting member located substantially at the apex of the cone, a resistor mounted near the base of the cone, and means for axially bending said contacting member into contact with said resistors.

7. A rheostat comprising two cooperating housings, an arcuate resistance element permanently disposed within each of said housings, a flat flexible centrally attached disc disposed in each of said housings concentric to said resistance element within each housing, a single shaft extending into the interior of said housings through one of said housings, two wipers attached to the end of said shaft within said housings, and a pressure shoe attached to each of said wipers for depressing the corresponding flexible disc in said housings.

8. In combination, a base member having a conical face, a resistance element disposed upon the conical face, a flanged sleeve extending axially through the member, and a thin metallic contact disc clamped between the flange of the sleeve and base member.

9. In combination, a base member having a conical surface, a resistance element disposed on the conical surface, a centrally apertured flexible disc overlying the resistance element, a flanged sleeve extending through the element and the member clamping the disc element rigidly to the member, a shaft extending through the sleeve, and an arm on the shaft cooperating with the disc to bring it progressively into contact with the element by flexure of the disc.

10. In combination, a base member having a conical surface and an axial hole, a resistance element disposed on said conical base and having stationary terminals secured to the member, a thin flexible contact member comprising a rim, spokes and hub portions overlying the element, said latter member having an integral terminal'secured to the base member, means clamping the hub to the base member and movable means for pressing the rim into engagement with the resistance element by fiexure of the disc.

11. In combination, a base member having a conical surface, a thin resistance element disposed upon the conical surface, a thin flexible metal disc supported on the base member and overlying the resistance element, said disc being supported against rocking and having limited rotary freedom about its axis and movable means engaging the disc at its periphery for deflecting the disc into engagement with the element, said disc being capable of limited angular motion as a whole under the action of said movable means.

12. The method of coupling two cupshaped members which comprises bringing the open rims into register, Wrapping a strip of wet fiber around the outside of the cups, clamping the ends of the wet strip together and drying the strip to produce contraction of the same.

13. In combination a cone-shaped resistor, a thin non-rotatable sheetlike contactor concentric with and disposed in a plane perpendicular to the axis of said resistor and adapted to be flexed into engagement therewith, a traveling presser member movable along the contactor and a substantially rectangular resilient pressure pad carried by the member and slidable along the contactor to press the same axially and resiliently into contact with the resistor. Y

14. In a rheostat, a base, a resistor having a frusto-conical surface and a contacting member adapted to be flexed into contact with the resistor, said cont-acting member comprising a substantially wheel-shaped member having a rim portion and a hub portion, and having spoke members extending between said portions said spoke members being graduated in section to vary the stiffness thereof.

15. In a rheostat, a base, a resistor having a circularly disposed contacting surface, a substantially disc-shaped contacting member, and means for flexing said contacting member into contact with the surface of said rellr sistor, said flexing means decreasing the effective radius of said member to cause a distributed slipping of said member on said resistor in a direction opposite that of the movement of said flexing means, and in definite proportion to said movement.

16. In a rheostat, a base, a resistor having a frusto-conical surface, a substantially flexible disc-shaped contacting member disposed 10 in a plane normal to the axis of said resistor surface, and rotatable means for flexing said contacting member into contact with the surface of said resistor, the relative stifiness of 'the contacting member providing a convex surface for contacting the convex surface of the resistor to provide a limited area of contact therebetween.

17. A rheostat comprising a resistor member and adjacent thereto a contact strip memher successive portions of which are adapted to progressively engage successive portions of the resistor member, the length of resistor engaged being substantially equal to but slightly different from the corresponding length of strip engaging it, means for producing a distributed slippage between the two as the point of engagement between the two is progressively advanced, said means comprising an arm adapted to progressively press points on one of said members into engagement with corresponding points on the other member, and means for rigidly holding a por tion of the pressed member against rotation, the distance from the pressedportions of the pressed member to the held portion thereof being substantially constant throughout the range of movement of the arm.

18. In a rheostat, a resistor comprising a strip of resistance material lying in a circular surface, a fiat circular spring metal contacting member comprising a hub portion and a circular peripheral portion normally lying in a plane above the strip and adapted to be flexed along its periphery into contacting engagement with the strip, and means for progressively pressing successive portions of the spring contacting member into contacting engagement with the resistance strip and simultaneously therewith producing between the two a slight rubbing movement distributed throughout the range of contacting engagement, said means comprising a movable arm pressing successive portions of the contacting member against the resistance strip and means for holding the hub portion of the contacting member rigidly against circular movement. In witness whereof, I hereunto subscribe my name this 3rd day of March, 1931. NEWTON C. SCHELLENGER. 

